1. Field of the Invention
The invention relates to a device for avoiding backlash (chattering) in gear trains of machines, particularly of printing machines. The machine has a first gear train, which has several mutually meshing gear wheels of which one is connected with a first drive. A tensioning device produces a torque in the normal rotational direction of the gear wheels between the beginning (first initial wheel) and the end (first end wheel) of the first gear train. The torque is sufficiently great so as to ensure a definite flank alignment of the gear wheels is each machine position.
2. Description of the Related Art
A device of the foregoing type is known from German published, non-prosecuted application 17 61 078. Its purpose is to coordinate the timing of the various cylinders of rotary printing machines, particularly of color printing machines, and to connect them without play. A torque is produced in the direction of rotation of the gear wheels by means of a tensioning device. The tensioning device is disposed between the beginning and the end of a gear train and helps avoids gear flank changes or an interruption in the alignment of the gear flanks. These occur, for instance, in the case of torque fluctuations during the operation of the printing machine. Such backlash, also referred to as chattering, can lead to a deviation from the desired coordinated rotational movement of the individual cylinders and even to register inaccuracies. The tensioning device known from the prior art is embodied as a longitudinal shaft which is connected with the beginning and the end of the gear train via suitable gear wheels and which, based on a defined torsion, effects a torque on the individual gear wheels of the gear train. The torque ensures a definite mutual flank alignment among the gear wheels. One of the gear wheels of the known device is connected with a drive, which also drives the rotary printing machine. The tensioning of the longitudinal shaft is very expensive in structural terms and the space requirements are substantial. Furthermore, it is only realized with a single motor drive.
In printing machines with multi-motor drives it is known to apply the main driving force with a motor while the other motor acts as a brake. This causes a definite flank alignment of the gear wheels in a corresponding gear train. Perfect printing results can only be achieved with extreme braking of the braking motor. This, of course, leads to corresponding losses.
It is further known in two-motor drives of printing machines to either accept inaccuracies, such as double print, from flank backlash due to unstable conditions or to provide a complicated control of the two motors. Such a control necessitates expensive and difficult measurements of the power flow. Even though, optimal printing results are not assured in each operating condition.